In recent years, the term “functional organic compound” has come and studies for using organic substances in electrical or optical devices have been actively pursued. Among such substances, luminous compounds (self-luminous compounds) having photo luminescence (PL) phenomenon are known.
An example of application of the luminous compounds is security ink. The security ink means an ink that its handwriting is invisible under visible light, but when being irradiated with ultraviolet rays, e.g. black light lamp, the handwriting emits light and one can read information recorded.
The security ink is used for the purpose of preventing forgery or reproduction or of recording secret information for purpose of confidentiality. For example, if the lot numbers, codes and the like of commercial goods are recorded with a security ink, it comes easy to trace the distribution channels of the commercial goods or to prevent the production or distribution of the forgery, and the fear of manipulation or damage of recorded information is reduced because the information is invisible under visible light.
The Patent Literature 1 (JP-A-2000-144029) discloses an ink composition with the following feature; being substantially invisible within the visible light region; containing a dye containing europium (Eu) complex having an emission center wavelength at 615±20 nm when being excited with ultraviolet rays and a polyvinyl resin; containing as a neutral ligand, at least a phosphonic acid type compound selected from a phosphine oxide compound, a phosphine sulfide compound and a phosphine compound; and containing not less than 94% of water and/or ethanol in a solvent.
The Patent Literature 2 (JP-A-2000-160083) discloses an ink composition for ink jet printing containing, as a luminous material, from 0.001 to 5% by weight of 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate europium chelate compound and also containing not less than 70% by weight of an alcoholic solvent as a solvent. Further, the Patent Literature 3 (JP-A-2003-26969) discloses an ink composition in which an europium compound represented by
is contained as a luminous material.
Further, with respect to an ink composition using the luminous material of europium complex, an aqueous ink for jet printing that becomes visible when being irradiated with ultraviolet rays (the Patent Literature 4, JP-B-Sho 54 (1979)-22336), a fluorescent europium complex for thermal transfer (the Patent Literature 5, JP-B-Hei 6(1994)-15269), a luminous ink composition containing an europium complex in which a bidentate ligand, e.g., bipyridine derivative or phenanthroline derivative, coordinates to europium (the Patent Literature 6, JP-A-Hei 3(1991)-50291), an ink composition which contains a tetra(benzoyl trifluoroacetonate)europium complex having an ammonium salt as a counter ion (the Patent Literatures 7, JP-A-Sho 64(1989)-6085 and 8, JP-A-Sho 64(1989)-26583) and the like are known as ink composition. On the other hand, the Patent Literature 9 Japanese Patent Kohyo 2005-518472 discloses the ink composition of europium complex having a trifluoromethyl group in the substituent of diketone being a ligand and in which the second ligand is sulfoxide. The Non-patent Literature 1, Mol. Phys. 101, 1037(2003) and the Patent Literature 10, JP-A-2005-41941, disclose a europium complex having a trifluoromethyl group at the substituent of diketone and its second ligand being sulfoxide, and its application of EL and LED. Further, the Patent Literature 11, JP-A-2003-129045 discloses a trisdiketonate europium complex having sulfoxide as the second ligand.
In considering practical use as a security ink, a handwriting or security information recorded with the ink is required to emit to a degree such that the ink is clearly visible when being irradiated with ultraviolet rays, e.g., a black light lamp (365 nm). However, the conventional luminous compounds or ink compositions is unsatisfactory in emission intensity or have a problem in stability when the ink contains alcohol as a solvent.
Therefore, there is a demand for providing a red luminous ink composition that exhibits excellent emission intensity such that a handwriting or image recorded which is substantially invisible within the visible light region can be visually recognized clearly.